Because the kilt is not a costume, it is a garment.
Sounds like your buddy is getting a little too uppity about clothing. You can point out equally silly fashion "rules" that he may be unconciously violating: wearing white shoes after Labor Day, wearing dark socks with shorts, mixing stripes and patterns, not matching his belt color to shoe color, wearing a adjustable pre-tied bow tie with a standup collar shirt, etc.

The fashion police are out there (and self-elected).

Now, If I could only get my hands on one of those Kilt police badges . . .